Recently, there are demands to reduce the inductance of signal pins of the semiconductor device in order to reduce simultaneous switching and power source bouncing generated at power source electrodes of the semiconductor device.
A ceramic package has been proposed to reduce the inductance of the signal pins. According to the ceramic package, leads are formed in a plurality of levels within a substrate so as to shorten the distance from a semiconductor chip to the signal electrodes, and the wiring layout is optimized. However, although the inductance of the signal pins can be reduced by the ceramic package, there is a problem in that the ceramic package is expensive compared to plastic packages.
Accordingly, a semiconductor device having a multi-level lead frame structure within the plastic package has been proposed. According to the multi-level lead frame structure, a lead frame having the leads, a lead frame having a stage on which the semiconductor chip is mounted, and a lead frame having the power source or ground electrodes are stacked, so as to increase the area of the power source or ground electrodes and enable reduction of the inductance of the signal pins.
Next, a description will be given of an example of a conventional semiconductor device having the multi-level lead frame structure, by referring to FIGS. 1 through 3. FIGS. 1 and 2 respectively show a cross section and a plan view of the semiconductor device, and FIG. 3 is a diagram for explaining the areas of parts forming the semiconductor device.
In FIG. 1, a semiconductor device 1 having the multi-level lead frame structure includes a semiconductor chip 2, leads 3, a stage 4, a first electrode member 5, a second electrode member 6, and a resin package 7 which is indicated by a one-dot chain line.
The semiconductor chip 2 is mounted on the stage 4. The first electrode member 5 is adhered on the lower part of the stage 4 via an adhesive tape 8. In addition, the second electrode member 6 is adhered on the lower part of the first electrode 5 via an adhesive tape 9. For example, the adhesive tapes 8 and 9 are made of polyimide.
On the other hand, as shown in FIG.3, the area of the first electrode member 5 is larger than that of the stage 4, and the area of the second electrode 6 is larger than that of the first electrode member 5. The shape of the adhesive tape 8 is approximately the same as that of the stage 4, and the shape of the adhesive tape 9 is approximately the same as that of the first electrode member 5.
Accordingly, in the assembled state, the stage 4, the first electrode member 5 and the second electrode member 6 form a pyramid like stacked structure as shown in FIGS. 1 and 2. If it is assumed for the sake of convenience that the first electrode member 5 is used as a power source electrode and the second electrode member 6 is used as a ground electrode, a power source pad 2a of the semiconductor chip 2 and the first electrode member 5 are connected by a Au wire 10a, and a ground pad 2b of the semiconductor chip 2 and the second electrode member 6 are connected by a Au wire 10b. The leads 3 and the electrode members 5 and 6 are respectively connected by Au wires 10c and 10d.
Therefore, the inductances of the signal pins can be reduced in the semiconductor device 1 because the lengths of the wires 10a through 10d which have relatively large inductances can be made short and the areas of the electrode members 5 and 6 can be made relatively large. In other words, the inductances between the pads 2a and 2b of the semiconductor chip 2 to the leads 3 can be reduced.
However, the stage 4 and the first electrode member 5 are connected via the adhesive tape 8, and the first electrode member 5 and the second electrode member 6 are connected via the adhesive tape 9. For this reason, the adhesive agent on the adhesive tapes 8 and 9 is decomposed when heated.
The production process of the semiconductor device 1 includes a heating process in which the temperature rises to a relatively high temperature. During such a heating process, an out gas is generated from the adhesive tape 9 and contaminates the electrode members 5 and 6. As a result, the adherence between the resin which forms the resin package 7 and the electrode members 5 and 6 becomes poor, and the wire bonding to the electrode members 5 and 6 becomes difficult and poor. In addition, if the effects of the out gas reaches the leads 3, the adherence between the resin and the leads becomes poor, and the wire bonding to the leads 3 becomes difficult and poor. The reliability of the semiconductor device 1 greatly deteriorates if the adherence becomes poor and the wire bonding becomes difficult and poor.
Furthermore, the adhesive tape 9 is expensive in general. Hence, when the adhesive tape 9 is used in the multi-level lead frame structure, the semiconductor device 1 becomes expensive.
Problems similar to the above are also generated due to the provision of the adhesive tape 8.